1. Field of the Invention
This invention relates to a device for calculating the coefficient of friction between a wheel and a road surface, for use in a brake control system or a drive power control apparatus for motor vehicles.
2. Description of Background Art
A brake control system has been known for motor vehicles such as motorcars or motorcycles, which calculates a slippage ratio for each wheel with respect to a road surface based on the running speed of the vehicle and the peripheral speed of each wheel, and applies the optimum braking force to the vehicle based on the calculated slippage ratios.
In the conventional brake control system, the hydraulic pressure of a brake is increased proportionally to the amount of depression of a brake pedal until a predetermined slippage ratio (hereinafter called a "target slippage ratio"), at which braking force becomes the maximum, is reached (hereinafter called "normal control mode"). After the target slippage ratio has been attained, the control mode is shifted to limiting increase in the hydraulic pressure of the brake so as to avoid a further increase in the slippage ratio (hereinafter called a "limiting control mode" or an "antilock control mode").
There is a relationship between a slippage ratio of each wheel with respect to the road surface and the coefficient of friction .mu. of the road surface, namely, the slippage ratio, at which a vehicle can stably be controlled, which differs between road surfaces to one having a high coefficient of friction (hereinafter called a "high .mu. road") and the other having a low coefficient of friction (hereinafter called a "low .mu. road").
A high .mu. road is usually assumed, in the conventional brake control system, in setting a target slippage ratio, and upon entering into the limiting control mode, the coefficient of friction .mu. between the wheel and an actual road surface is then calculated to change over the target slippage ratio to another, thereby to enlarge the range in which the normal control mode is applicable. There has been disclosed in Japanese Patent Application Laid-Open Publication No. 62-194963 such a technical idea of detecting the coefficient of friction .mu. between the wheel and the actual road surface at the time of shifting the control modes from normal to limiting.
However, in the conventional brake control system described above, there was the danger of setting a suitable target slippage ratio too late, namely, after the control mode shift.